Acid- base homeostasis

Question 1

How is acid buffered in the body normally

Answer 1

• Proteins eg. Hb
  
  • Combines with H+ to prevent them from being free
• HCO3-
  
  • Measurements reflect acid-base balance

Question 2

What measurement must be calculated in acid base disorder

Answer 2

HCO3- using Henderson-Hasselbach equation1

Question 3

Role of standard bicarbonate ? Mixed disorder?

Answer 3

• [HCO3- ] if pCO2 were reference range
• Only deranged when metabolic problem*is present
  
  • Resp ⇒ ref range
  • Met disorder ⇒ equivalent to actual bicarb (since pCO2 not really the problem)
  • Mixed disorder → significant difference from actual bicarb

Question 4

What is base excess

Answer 4

Amount of H+ per L of blood required to return [H+] to reference range pCO2 (~5.3 kPa)

• Only deranged when metabolic problem is present
  
  • Resp ⇒ ref range
  • Met acidosis ⇒ -ve
  • Met alkalosis ⇒ +ve

Question 5

Anion gap

Answer 5

• Anion gap
  
  • To narrow differentials of metabolic acidosis ⇒ depends on what the HCO3- are replaced with:
    Na+ -Cl- HCO3-
    Elevated : Cl- does not change, HCO3- replaced by anions corresponding to lactate, keto acids etc.: DKA, lactic acidosis, aspirin overdose, methanol poisoning, renal failure
    Normal : Cl- replace HCO3- eg. diarrhoea, RTA, High intestinal fistula output

Question 6

Main mechanism of maintaining pH ( what pumps involved)

Answer 6

Na in , exchange with H+ out, HCO3- in

Question 7

What is the fastest means of acid removal

Answer 7

buffers

Question 8

What is role of Total CO2

Answer 8

Similar to HCO3- as 95% is HCO3-, measured by enzymatic essay, acts as rough guide suggesting need for blood gas if deranged

Question 9

Why is there hypokalemia in alkalaemia

Answer 9

Shift of K+ from plasma into cells

Question 10

Why is there likely to be hypokalalemia in cirrhosis

Answer 10

Hyperaldosteronism secondary to cirrhosis ⇒ liver not producing enough albumin and results in more production of Na+→ urinary loss of K+

Question 11

does asthma cause respirator alkalosis or acidosis

Answer 11

alkalosis

Question 12

Does PE cause alkalosis or acidosis

Answer 12

alkalosis

Question 13

Does COPD cause alkalosis or acidosis

Answer 13

acidosis(infective exacerbation) and alkalosis

Question 14

Does pneumonia cause alkalosis or acidosis

Answer 14

Acidosis

Question 15

What acute neurological problems can cause acidosis

Answer 15

GBS, MG, Opiates

Question 16

What chronic neurological problems can cause acidosis

Answer 16

MND, myopathy

Question 17

Effects of acute hypocapnia

Answer 17

(a) Cerebral vasoconstriction ⇒ Light-headedness common in panic attacks, confusion, syncope, fits

(b) Fall in ionised calcium since calcium less soluble within plama ⇒ perioral, peripheral paraesthesia commonly at lips

CV⇒ increased HR, vasoconstriction (chest tightness, angina in those with hisotry of CAD)

Question 18

Effect of acute hypercapnia

Answer 18

SOB( though drive impaired in chronic retention)
Neurological ⇒ anxiety, coma, headache, extensor plantars, myoclonus
CV⇒ systemic vasodilation (CO2 is potent vasodilator)

Question 19

causes of metabolic alkalosis

Answer 19

(1) Decreased H+
Upper GI ⇒ Vomiting most common (rich in H+ ions)
Renal loss of ions

(a) Hypokalemia eg. secondary to loop diuretic
(b) Primary hyperaldosteronism

(2) Increased HCO3-
Iatrogenic ⇒IV sodium bicarbonate

Question 20

Causes of metabolic acidosis

Answer 20

(1) Increased H+
Over-production
Lactic acidosis ⇒ Tissue hypoxia (sepsis, anaemia, major haemorrhage, cardio resp arrest, peri vasc disease, gen seizure)

Ketoacidosis ⇒ Diabetic, starvation, alcoholic

Poisoning(more unusual) ⇒ salicylate, methanol

Inherited metabolic disorders( usually present in early infancy)

Impaired excretion
Global loss of renal function (renal failure) ⇒ AKI,CKD, AKI on CKD

RTA (specific to tubules)⇒ Types 1 and 4

(2) Decreased HCO3-
Renal ⇒ RTA type 2
GI ⇒Severe diarrhoea, high output small bowel fistula (loss of fluid from gut that is HCO3- rich)

Question 21

Effects of metabolic acidosis

Answer 21

Cardiovascular ⇒ negative inotropic effect if severe

Oxygen delivery ⇒
Acutely H+ causes R-shift of oxyHb dissociation curve and facilitates O2 delivery

After several hours ⇒ Dysfunction of Hb, H+ reduces 2,3-DPG causing L-shift of curve, impairing O2 delivery

Nervous system ⇒ impaired consciousness

K+ homeostasis ⇒ leakage from cells causes high plasma [K+] and [Ca2+] (hyperkalaemia common with metabolic acidosis acutely , if sustained ⇒ may have renal loss

Bone ⇒ Demineralisation of bone to buffer chronic acidosis as calcium more easily dissolve → decalcification

Question 22

Why does Hypokalemia worsen alkalosis (met)

Answer 22

Since K+ usually excreted in return for Na+ absorbtion ⇒
Reabsorption of Na+ for excretion of H+ also occurs , more significant if already low K+